Poor nutritional status is a major concern for children being treated for an oncology condition. It is vital for optimal prognosis that nutritional concerns are recognised and treated promptly. The majority of oncology treatment centres only use simple methods to screen for poor nutritional status; however, it is unknown whether these simple methods accurately recognise poor nutritional status. We aimed to determine whether commonly used simple nutritional assessment variables could accurately identify poor nutritional status in children being treated for oncological conditions. This cross-sectional study measured height, weight, mid-upper arm circumference, triceps skinfold, albumin levels and body cell mass in forty children being treated for an oncological condition. To determine whether commonly used nutritional variables were suitable determinants, the variables were compared against the reference measure of body cell mass index (BCMI) Z-scores. Using the BCMI Z-score cut-off of − 1·65, 48 % of the study population were considered poorly nourished. Correlational analysis showed that there was no significant biological relationship between the BCMI Z-score and the simple parameters. When divided into two groups based on the nutritional status indicated by the BCMI Z-score, the independent t tests between the well-nourished and malnourished groups demonstrated that there was a significant difference in the BMI Z-score (P = 0·01) between the groups. No simple nutritional measures were found to accurately identify poor nutritional status in children being treated for oncological conditions.